Dehydration and heat-related illness has long been a common cause of mortality in outdoor situations. Dehydration is defined as sweat, urine or respiratory water loss. It results from the voluntary restriction of intake of water. Thirst is not a good indicator for when to replace water. This has been seen in marathon runners who are accustomed to running long distances. When endorphins take over, the thirst mechanism is diminished or disregarded entirely. So being thirsty is not a good indicator for how dehydrated one might be. Dehydration also decreases work performance.

You can lose 2% of your body weight before thirst is initiated, which may get worse as the severity of dehydration increases. With 4%-6% body water loss, you are anorexic, impatient and you have symptoms of headache. With 10% loss, you have dizziness and cyanosis. You also become light-headed and have syncopal episodes. With 12% loss, you have difficulty swallowing and you require assistance in re-hydration. 15%-25% water loss is lethal.

Don’t think that you only need to be properly hydrated in high temperatures, you must also hydrate when it is cold and at high altitude. At high altitudes, water loss can be very similar to those in the desert. Some very rough estimates for water loss in the cold is approximately 2 quarts per day at moderate exercise levels and up to 2 gallons in severe cold with heavy exercise. At altitudes above 10,000 feet, even more than that.

When exercising you should drink a little all the time at least 3 oz. every 15 minutes and a lot at meals and at bedtime. A quick rule-of-thumb indicator of being properly hydrated is to be able to urinate pale yellow. If you can’t remember the last time you urinated, you are significantly dehydrated. Drink by your watch in hot environments. Forced drinking in the absence of thirst saves lives in the heat.

There can also be a cumulative effect of dehydration and heat exhaustion which can occur over days, and is not recognized, especially in athletics. The cumulative effect is usually not taken into consideration.